The present invention relates in general to electrical circuit protection materials and methods, and, in particular to novel electrical overstress materials capable of conforming to predetermined shapes, as well as novel methods and processes for producing such materials. Apparatus for practicing the novel methods and processes, as well as both devices and systems which utilize such novel materials, are also disclosed.
The following specification and claims, read together with the accompanying drawings, are presented merely to teach examples and embodiments of the present invention, and should not be read or construed as limiting the proper scope of the claimed invention.
Conventional surge protection devices include fuses, varistors, Zener diodes, spark gap and thin-film devices, along with electronic filter circuits. Each of these devices and circuits is shown to have one shortcoming or another in countering the effects of fast-rise, broad frequency spectrum, transient voltage or current surges, or in recovering from such repeated surges.
More specifically, conventional surge protection devices are too slow in reaction times to provide protection for many modern delicate circuit elements, have inadequate voltage dissipation capacity to protect such delicate circuit elements from large electrical stresses, and are subject to destructive breakdown after a single high voltage surge, thereby leaving the circuit unprotected from any subsequent surge.
The concept of clamping voltage is also important in understanding electrical overstress materials. Clamping voltage, in relation to the nonlinear resistance characteristics of a device, is usually considered as being the overall or bulk resistance of a surge protection device or material in relation to a voltage applied to it. For applied voltages below the clamping voltage, a nonlinear resistance device, such as a varistor, offers the circuit a high resistance approaching the characteristics of an insulator, while, for applied voltages substantially above the clamping voltage, the device offers the circuit a substantially reduced resistance capable of shunting transient electrical surges to ground.
The prior art contains teachings of electrical resistance composites intended for purposes similar to that of the present invention, but they differ from the present invention and do not accomplish the same results.
U.S. Pat. No. 4,977,357 issued to Shrier discloses an electrical overstress material having conductive particles uniformly dispersed in an insulating matrix or binder to provide material having non-linear resistance characteristics that are determined by the interparticle spacing within the binder as well as by the electrical properties of the insulating binder. U.S. Pat. No. 4,726,991 issued to Hyatt et al. discloses specific materials intended for electrical overstress protection and provides both a general overview of the problem and a survey of the known prior art. Hyatt et al. proposes a material for electrical overstress protection that has a nonlinear resistance as the voltage across it varies. The material comprises a matrix containing a mixture of small conductive and semiconductive particles coated with inorganic insulating material that permits nonlinear conduction between the particles in response to electrical transients by what is believed to be the quantum mechanical tunneling of electrons in the matrix materials. In a preferred embodiment the material includes a binder or packaging material in which the particles are generally homogeneously mixed.
U.S. Pat. No. 4,359,414, issued to Mastrangelo, discloses an electric current regulating junction capable of being electrically switched between low resistance and relatively high resistance states by the application of a relatively low current pulse and that is useful as a computer memory storage element. The material comprises a normally insulative, electrically activatable composition disposed as a layer 0.1 to 2,540 microns in thickness that has an electrical resistance greater than 10.sup.8 ohms through its thickness. The composition consists essentially of 10 to 85 volume percent of a substantially linear, unitary polymeric binder having a glass transition temperature of at least 100 degrees Centigrade and is selected from aromatic polimides, aromatic poly(amides-imides), aromatic poly(ester-imides) and aromatic polyamides. The binder has 15 to 90 volume percent of particles of aluminum substantially homogeneously dispersed in it. The aluminum particles are stated to have electrically conductive metallic interiors and thin electrically insulative surface coatings of aluminum oxide sufficient to impart electric resistance between any two or more particles in contact with each other. However, this patent neither states nor suggests the use of semiconductive particles, whether they be coated or not, nor does it provide for its junction material automatically returning to its high resistive state once an initial triggering pulse is shunted to ground.